The major global consuming regions for cryogenic liquid CO2 are the United States, Western Europe, and Japan. The United States is the largest consumer at about two thirds of the total global amount. Globally, the major cryogenic liquid end-use is for food processing and carbonated beverage production.
Liquid CO2 is usually recovered as a byproduct from bio-ethanol production and catalytic steam reformation of natural gas followed by the water-shift reaction to produce ammonia. In the United States, there is increasing CO2 availability from bio-ethanol production due to the growing need for clean transport fuels and chemicals, and decreased availability due to declining ammonia production caused by high natural gas prices.
New bio-ethanol CO2 sources are increasingly located west of the Mississippi River due to corn feedstock availability, whereas significant liquid CO2 demand is in the eastern densely populated Northeastern, Southeastern, and Southern states. This has created a supply and demand balancing dilemma for both producers and consumers of liquid CO2 
Most of the Kraft paper pulp mills in the United States are located in regions of high liquid CO2 demand. If new CO2 producing sources could be created within those high need regions, a supply and demand problem would be solved.
Further, many global pulp and paper mills have precipitated calcium carbonate (PCC) “satellite plants” that supply this important paper filler to papermakers. PCC production requires industrial lime feedstock and sourcing CO2 contained in adjacent pulp mill lime mud calciner off-gases. There are supply reliability, quality, and cost issues associated with this approach such that a more reliable, higher quality, and consistent CO2 feedstock source would be attractive.
Also, there will be a significant future need for CO2 within Kraft pulp mills to precipitate organic lignins from aqueous “black liquor” fuel streams normally supplied to the chemical recovery boiler. This de-bottlenecks the boiler while creating a valuable, new “carbon-neutral” biomass derived fuel that can displace fossil fuels.
The Kraft pulp and paper industry is also a major energy consumer, with the majority of that need being met by low cost, carbon-neutral, biomass and biomass related fuels. The conventional lime mud calcination process has, however, not easily been converted to biomass fuels and remains a conspicuous consumer of high cost, greenhouse gas emitting fossil fuels. In the United States, there are 150 Kraft pulp mills with a typical 1000 air dried tons per day (adtpd) bleached pulp mill requiring 320 tpd of calcined lime mud with an annual natural gas and oil consumption of approximately 625 billion Btus. At 2007 energy prices this is approximately US $4.0 million per year, per mill, or $600 million for all US mills.
It would be useful to regenerate concentrated CO2 from more dilute CO2 sources as the need for large scale “greenhouse-gas” capture and sequestration projects develops. One capture process utilizes sodium hydroxide to capture dilute CO2 globally present in ambient air. Another uses sodium hydroxide to capture more concentrated CO2 present in electric utility or industrial boiler stack gas streams. Both capture schemes would benefit from a low-cost process to regenerate concentrated CO2 from lime mud made in the associated re-causticizing process that produce the sodium hydroxide scrubbing liquor.
In the Kraft paper pulping process, cellulosic wood chips are mixed with aqueous cooking liquor (a.k.a. “white liquor”) composed primarily of sodium hydroxide (NaOH), sodium sulfide (Na2S), sodium carbonate (Na2CO3) and sodium sulfite (Na2SO3). This mixing occurs in a “digester” vessel at a temperature and pressure satisfactory to separate the cellulosic fiber from the natural lignins that bind such fibers.
The liberated fiber is separated from the resultant “black liquor” and is subsequently washed, bleached (or remains unbleached) and is eventually transformed into numerous paper grades.
The separated black liquor contains, aside from the original white liquor chemicals, lignins and other organic matter that previously bound the cellulosic fiber. In order to recover and recycle these costly pulping chemicals, as well as produce valuable pulp mill steam and power from the contained organic lignins, the black liquor is concentrated in multiple-effect evaporators and delivered as a concentrated fuel to a “chemical recovery boiler.”
This recovery boiler combusts the organics under unique oxidizing/reducing conditions to both produce high-pressure steam and a molten inorganic ash (“smelt”) consisting primarily of Na2S and Na2CO3. The co-produced high-pressure steam is subsequently exhausted via a steam turbine/generator to produce mill power and lower pressure mill process steams.
The smelt is drained from the recovery boiler and quenched in water to create “green liquor.” This green liquor is subsequently clarified and filtered to remove insoluble impurities whereupon it is delivered to the “slakers” to initiate conversion of the dissolved Na2CO3 into NaOH required in the white liquor. This slaking process utilizes calcium oxide CaO (a.k.a. re-burned lime) to convert Na2CO3 into NaOH via the following two consecutive reactions:CaO(s)+H2O→Ca(OH)2(s)  1)Na2CO3(aq)+Ca(OH)2(s)→2NaOH(aq)+CaCO3(s)  2)
The slaker product slurry, consisting of all the chemicals involved in reactions 1 and 2, is fed to subsequent re-causticizers where reaction 2 nearly proceeds to completion with some residual Na2CO3 remaining in the white liquor. The resultant white liquor mix of NaOH, Na2S, Na2CO3, and Na2SO3 is physically separated from the precipitated calcium carbonate (CaCO3) and recycled to the digester to initiate the pulping process.
The precipitated CaCO3 (also known as lime mud) is further water washed and filtered to recover as much white liquor as economically possible before being fed to a rotary kiln calciner which converts the mud into re-burned lime, or “calcine”, (CaO and impurities) for recycle to the slakers. During the washing/filtering process, trace amounts of residual Na2S are air oxidized into more stable sodium thiosulfate (Na2S2O3) to reduce noxious total reduced sulfur (TRS) compounds which can be created in and emitted by the rotary kiln from Na2S.
The highly endothermic lime mud calcination reaction typically occurs in a rotary kiln, although fluidized bed calciners have also been utilized. Use of an external lime mud flash drying (LMD) process, when combined with the rotary kiln, creates the current “state-of-the-art” optimized energy consuming lime mud calcination process.
The first fluidized bed (“FluoSolids”) lime mud calcination process was commercially introduced in 1963. It initially gave significant competition to rotary kilns due to its relatively lower fuel consumption, higher product quality, and compactness. It fell into disuse, however, as rotary kiln/LMD technology re-captured the fuel economy lead and FluoSolids installations experienced operability issues and an inability to economically operate at the high unit capacities required by a “world-class” Kraft pulp mill.
The kiln's primary endothermic (TR=25° C.) calcination reaction is:CaCO3(s)→CaO(s)+CO2(g),+42.5 Kcal/gm mole (891,764 Kcal/metric ton CaO)  3)
The rotary kiln calcines the mud between 1000° C. (1832° F.) and 1200° C. (2192° F.) and at CO2 partial pressures well below the atmospheric pressure equilibrium concentration for these temperatures. This produces a rebumed lime having the best physiochemical properties suitable for subsequent slaking and efficient recausticizing.
Due to the high calcination temperatures, and so to not contaminate and/or upset the re-causticizing process with inorganic impurities, either high-cost oil and/or natural gas fuels are utilized as kiln fuel. Low-cost solid fuels such as biomass, waste water treatment plant (WWTP) sludge, coal etc. are typically not used as-is due to their contaminating ash content. Wet WWTP sludge and biomass have the added penalty of lower adiabatic flame temperature.
Accordingly, while many energy-intensive pulp mill operations have converted to low-cost waste and biomass fuels for energy production since the 1970s, the rotary kiln remains a conspicuous consumer of premium liquid and gaseous fuels. While advances have been made to reduce this premium fuel consumption, it still remains between 1.4 (with LMD) and 1.7 million Kcal/metric ton calcine dependent on initial mud moisture content, calciner capacity, fuel type, product lime availability, and installed energy conservation features.
Due to technology limitations, attaining future significant fossil fuel consumption/cost reductions in the rotary kiln/LMD calcination process appears difficult. There is, however, wasted energy within the rotary kiln/LMD calcination process that could be recovered with the proper technical approach. At higher lime mud solids concentration the calciner's exit gas temperature increases. If a counter-current heat transfer process (such as a rotary kiln) were thermally balanced the exit gas temperature would remain constant as fuel input was reduced to compensate for the decreased water input.
Such energy efficiency, however, is not possible with the rotary kiln/LMD calcination process since a very large non-variable fuel amount is required to provide the constant endothermic heat-of-reaction enthalpy and also heat reaction products (CaO and CO2) to the calcination temperature. This non-variable fuel input has associated gaseous fuel combustion products from which heat is recovered via counter-current contact with dried lime mud solids in the kiln pre-heat section using densely packed hanging chains as heat transfer surface. In this manner, dried lime mud is pre-heated before it enters the following kiln calcination stage. This reduces fuel consumption.
The reduced temperature gaseous combustion products (and released CO2) leave the kiln pre-heat section and enter the kiln drying section where these gases' enthalpy content evaporates incoming lime mud water content. Older kilns have chains within the kiln drying section to improve gas-to-water heat transfer. Newer kilns with an LMD do not have drying section chains and are easier to control and operate. As previously stated, as lime mud solids content increases the need for drying enthalpy decreases. The following kiln pre-heat section, however, has insufficient chain heat transfer ability to absorb available energy from the combustion products and CO2 associated with the aforementioned non-variable fuel component and transfer it into the dried solids entering from the drying zone. This unabsorbed, unwanted combustion products and CO2 enthalpy exits the system as higher LMD outlet gas temperature when high solids lime mud is used. Over the last thirty years, improvements in lime mud filtration and washing have increased filter cake solids content from 70% to over 85%, resulting in significant fuel savings and improved white liquor recovery. Unfortunately, the current rotary kiln/LMD technology is limited in the ability to economically respond to this fuel saving opportunity and will become less fuel-efficient as filter cake solids content further increases.
The less utilized fluidized bed calcination process never featured a solids pre-heat section, and wastefully dissipated this excess heat via a water spray cooler to control lime mud flash dryer exit temperature. Designs have also been proposed to address this dilemma by inserting a waste heat boiler in place of the spray cooler step, but this may never be commercialized due to the high surface fouling characteristics of calciner exit gas caused by the presence of low eutectic melting point Na2CO3/Na2SO4 mixtures.
It would, therefore, be beneficial to provide a process whereby fuel combustion products could be separated from gaseous calcination reaction products (CO2) such that the excess heat contained in the combustion products could be viably extracted as process steam without the presence of heat transfer fouling mixtures such as Na2CO3/Na2SO4. This is not possible within the body of a rotary kiln however the disclosed invention, with separated combustion and calcination stages, addresses this need.
Concurrent with these needed fuel reduction efforts, all mills must control the amount and toxicity of gaseous, liquid, and solid wastes expelled. Many of these emissions have been reduced or eliminated thanks to better manufacturing practices but WWTP sludge (cellulosic, organic, and inorganic matter from waste water treatment) remains a costly disposal problem since it must ultimately be placed in a landfill. As previously discussed, WWTP sludge cannot be used in existing rotary kiln representing a lost opportunity to conserve fossil fuels.
Safe disposal of non-condensable waste mill gas (NCGs), which are typically combusted in the recovery or power boiler, or more likely, the rotary kiln lime mud calciner. While NCG combustion in rotary kilns has been widely practiced, operability problems (kiln deposit “ringing”, SO2 “blow-through”, etc.) persist at most mills Accordingly, stand alone NCG incinerator/boilers that raise steam and scrub sulfurous emissions are increasingly used. These incinerator/boilers, however, are not always available when NCGs are produced so a back-up disposal means is desirable.
Numerous advances have been previously made related to various aspects of lime mud and limestone calcination. U.S. Pat. No. 2,212,446 teaches limestone calcination in a 100% steam atmosphere (a claim of the disclosed invention) using an indirect heated rotary calciner. U.S. Pat. No. 2,700,592 teaches using moving media heat transfer (MMHT) between an endothermic fluidized bed process and an exothermic fluidized bed sulfide ore roasting process. U.S. Pat. No. 2,738,182 teaches fluidized bed calcination of Kraft pulp mill lime mud including recycling finely ground calcine product into a calciner bed to control agglomeration. U.S. Pat. No. 3,961,903 teaches a spray dryer to dry lime mud using multiple hearth calciner off-gases as the drying medium prior to feeding the dried mud to the calciner. U.S. Pat. No. 3,991,172 teaches direct combustion products calcination of fine limestone by passing the limestone through a fluidized bed of a “granular heat carrier medium”. U.S. Pat. No. 4,321,239 teaches using multiple spray dryers to dry lime mud using multiple hearth calciner off-gases as the drying medium prior to feeding the dried mud to a calciner. U.S. Pat. No. 4,389,381 teaches using MMHT by passing fine limestone through an inert heat carrier contained in an endothermic fluidized bed and using a coal fueled exothermic fluidized bed to re-heat the heat carrier. Ash is separated from the re-heated heat carrier prior to calcination. Calcination is accomplished in an air atmosphere of unspecified composition. U.S. Pat. No. 4,606,722 teaches a solid fuel gasified external to a rotary kiln lime mud calciner with the syngas used as calciner fuel. A vitrified gasifier ash is mixed with calcine and removed in the slaker. U.S. Pat. No. 4,631,025 teaches direct injection of a solid fuel (petroleum coke) into a fluidized bed lime mud calciner. U.S. Pat. No. 4,707,350 teaches calcination of fine limestone in an electrically heated fluid bed calciner fluidized in a 100% CO2 atmosphere with recovered CO2 as the fluidizing gas. U.S. Pat. No. 4,760,650 teaches indirect steam heated drying of lime mud in a steam atmosphere prior to feeding the dried lime mud into a fluid bed calciner. The steam is generated from calciner off-gas. U.S. Pat. No. 5,110,289 uses a separate flash dryer to dry Kraft pulp mill lime mud using rotary calciner off-gases as the drying medium. U.S. Pat. No. 5,230,880 teaches calcination of fine limestone in an electrically heated fluid bed calciner fluidized in an air atmosphere. The fine limestone is passed through a bed of coarser calcined limestone particles that act as a heat transfer media between the fine limestone and the electric heaters. U.S. Pat. No. 5,354,375 describes a lime mud calcination process using a shaft kiln to process pelletized lime mud in a counter-current fashion using direct firing of oil or natural gas fuel. U.S. Pat. No. 5,378,319 describes a lime mud calcination process using an electrically heated microwave belt oven to process lime mud in a counter-current fashion using a counter-current air sweep. U.S. Pat. No. 5,644,996 teaches a technique to cool freeboard gases in a fluidized bed lime mud calciner to below 500° C. (932° F.) to minimize freeboard scaling when the calciner fluid bed is between 875° C. (1607° F.) and 1000° C. (1832° F.). The injected coolant is the entire amount of wet lime mud. U.S. Pat. No. 5,653,948 teaches an indirectly heated fluid bed calciner using electricity or oil/gas firing to calcine very fine limestone particles. The limestone is injected beneath a coarser limestone bed that acts as the heat transfer medium. U.S. Pat. No. 5,711,802, teaches a technique to reduce the LMD inlet gas temperature from a rotary kiln lime mud calciner to between 400° C. (752° F.) and 600° C. (1112° F.); eliminates dryer scaling and reduces kiln dust carry-over. U.S. Patent Application Publication No. 2006/0039853 teaches a process to separate CO2 from utility boiler stack gases with an “activated” CaO sorbent and then separately re-generating the sorbent and recovering the CO2 in a steam blanketed vacuum calciner.